debian: add maintscript helper to remove usr.lib.snapd.snap-confine in snap-confine

[mvo5]

The old snap-confine package used to own the usr.lib.snapd.snap-confine
conffile. So we need to put the maintscript helper into the transitional
snap-confine package to get this file really removed.

LP: 1682023

[zyga]

The mysteries of dpkg continue to amaze

[awhitcroft]

I am able to reproduce this issue locally using an old 2.22.6 and the latest 2.23.6 binaries Using packages built with this fix applied the errant profile is correctly removed during reinstall:

Removing obsolete conffile /etc/apparmor.d/usr.lib.snapd.snap-confine ...

[jdstrand]

LGTM